At the present time multiple glazed units are employed to minimize heat loss from a structure while providing visual access into and out of the structure. The multiple glazed units have evolved through the years and included in that evolution are units having a dead air space between the sheets, units having an insulating gas in the space, units having a vacuum in the space, and units having an environmental coating e.g. low emissivity coatings on the glass sheets. Of particular interest in the discussion are the units that have a vacuum in the space between the sheets e.g. types taught in U.S. Pat. Nos. 3,974,822; 3,990,201; 4,393,105; 4,586,289; 4,683,154 and 4,786,344.
Units having a vacuum in the space are of particular interest because the evacuated space minimizes or nearly eliminates the conduction and convection heat transport. These types of units constructed according to the teachings of the prior art, however, have drawbacks. More particularly, the vacuum between the glass sheets causes the sheets to bow or deflect toward one another; the excess bowing can result in the sheets fracturing or contacting one another. When the sheets contact one another a heat conduction path is established through the unit by direct conduction, thereby reducing the thermal efficiency of the unit. Another concern is providing an edge seal that maintains the vacuum in the space between the sheets.
U.S. Pat. No. 4,393,105 teaches a unit that has the space between the sheets filled with a low heat-loss gas or left under vacuum. The patent does not positively teach a technique to keep the glass sheets from contacting one another. Further, the patent implicitly teaches a rigid frame to space the sheets. A drawback with a rigid frame is that it does not accommodate sheet movement and may result in seal fracture.
U.S. Pat. No. 3,974,822 teaches the use of spacers made of cork or plastic to maintain the sheets apart; however, there are no teachings therein to provide an edge seal that maintains the vacuum in the air space for an acceptable period of time. U.S. Pat. No. 4,586,289 teaches a vacuum insulating window and reflector for controlling the heat gain and loss of a structure. Although the unit provides for separators to keep the sheets apart, the edge seal of the unit is merely referred to as a sealant material in the space between the glass sheets and the outer channel. The channel appears to be rigid and therefore cannot accommodate movement of the glass sheets due to thermal differences. The result is that the edge seal will fracture allowing air into the space between the sheets.
U.S. Pat. No. 3,990,201 teaches a vacuum unit having a pair of glass sheets separated by insulating spacers and an O ring between the sheets. The sheets are mounted in a U-shaped channel and have mica spacers therebetween. The unit is not a sealed unit, but uses an O-ring to provide a seal. There are no provisions for preventing marring of the sheets as they move relative to one another over the spacers.
U.S. Pat. Nos. 4,683,154 and 4,786,344 each teach a vacuum unit having spacers between the sheets and the edges sealed to a glass filament or the edges of the sheets sealed together. These types of edge seals do not provide for expansion of the units that may occur due to thermal difference of the sheets.
From the above, it is clear that a vacuum unit that does not have the limitation or drawback of the vacuum units taught in the prior art is desirable.